This project will investigate the interactions of subpopulations of thymus-derived lymphocytes (T cells) in immunity and tolerance. Special emphasis will be given to a subset of naturally proliferating T cells in order to functionally characterize their role in various cell-mediated immune reactions, i.e., graft-versus-host reactions, mixed lymphocyte interactions, and responses to T cell mitogens, and to determine the relationship of these cells to classical long-lived, nonproliferating T cells. This project will also investigate the cellular mechanisms which control both the induction and expression of T cell tolerance in contact hypersensitivity to DNFB and TNCB in mice. These studies will emphasize (1) characterization of the cells involved, i.e., alloantigens such as Ia and Ly, (2) the genetic restrictions for their interaction, (3) identification and characterization of soluble factors, and (4) elucidation of the mechanism(s) of T cell-mediated immune regulation.